Common known examples of methods for applying a preload to a bearing in the axial (thrust) direction include a fixed-position preload structure incorporating a screw or the like, and a constant-pressure preload structure incorporating a spring or the like. Patent Document 1 discloses a bearing device comprising a fixed-position preload structure, and Patent Document 2 discloses a preload device comprising a constant-pressure preload structure.
A device comprising a cup-shaped flexible externally toothed gear is a known example of a hollow strain wave gearing unit comprising a device hollow part passing through in the axial direction. Patent Document 3 discloses a hollow actuator having a configuration in which a hollow motor is integrally formed in the axial direction with a hollow strain wave gearing reducer comprising a cup-shaped flexible externally toothed gear.
The wave generator of the strain wave gearing reducer disclosed in Patent Document 3 comprises a hollow rotating shaft, an ellipsoidally contoured cam plate (plug) integrally formed on the external peripheral surface portion of the hollow rotating shaft positioned on the inner side of the flexible externally toothed gear, and a wave generator bearing attached to the ellipsoidal external peripheral surface of the cam plate. A shaft end part of the hollow rotating shaft positioned on the inner side of the flexible externally toothed gear is supported by a bearing held in a bearing holder mounted on a boss of the flexible externally toothed gear. A preload is applied to the bearing held in the bearing holder from the side opposite the boss, and the preload is applied by a constant-pressure preload structure incorporating a coil spring.